A display device for causing an observer to observe a two-dimensional image formed by an image forming device as an enlarged virtual image by a virtual image optical system is well known from, for example, Japanese Patent Application Laid-Open No. 2007-309045. As illustrated in the conceptual diagram of FIG. 34, the display device includes an image forming device 911 including a liquid crystal display device having a plurality of pixels arranged in a two-dimensional matrix, a lens system 114 on which an image emitted from the image forming device 911 is incident, and a light guide device 120 on which an image emitted from the lens system 114 is incident, which guides light, and which emits the light. The light guide device 120 includes: a light guide plate 121 from which light incident on the light guide device 120 is emitted after the light is propagated by total reflection inside the light guide device 120; a first deflecting unit 122 for reflecting light incident on the light guide plate 121 such that the light incident on the light guide plate 121 is totally reflected inside the light guide plate 121; and a second deflecting unit 123 for emitting the light propagated by total reflection inside the light guide plate 121 from the light guide plate 121. In addition, if such a display device constitutes, for example, a head mounted display (HMD), the weight and size of the device can be reduced. Each of the first deflecting unit 122 and the second deflecting unit 123 includes, for example, a reflection type volume hologram diffraction grating. In addition, by displaying an image on the display device, an observer 20 can view the displayed image superimposed on an image of an outside world. Incidentally, for the other reference numerals in FIG. 34, refer to reference numerals in a display device described in Example 1.